Project Summary/Abstract Transposons are prolific genetic parasites infiltrating >45% of the human genome and are major proportions of all animal genomes. Our lab studies how the Piwi/piRNA pathway, an arm of the RNA interference (RNAi) system, recognizes and silences transposon transcripts to preserve genome stability and fertility. Our lab specializes in examining the interplay and dynamics between transposons and the Piwi/piRNA pathway with the goal of learning better how to prevent transposons from proliferating and negatively affecting animal health. To fundamentally uncover the regulatory mechanisms between animal genomes and transposons, we are deploying genomics, biochemical and small RNA analytical approaches on the Piwi/piRNA pathway. The major focus of this project is to study a hyper-mobile Har-P-element transposon variant that we discovered is the ammunition for P-transposase to drive the infertility syndrome of gonadal dysgenesis (GD) in Drosophila. Our project?s advantage relies on unique Drosophila strains and cell line assays developed in the Lau lab to study how the Har-P-element is regulated by piRNAs, how the enhancer of the Flamenco piRNA cluster can generate piRNAs from Har-P-elements, and how do Har-P piRNAs regulate intron splicing. This project will achieve the following aims in this project: Aim 1: Determine the mobilization capacity and paternal silencing mechanism of the Har-P-element variant, a hypermobile transposon that drives severe GD. Aim 2: Dissect the transcriptional regulation of P-element piRNAs coming from the Flamenco piRNA cluster and the mechanistic link between piRNA silencing and intron splicing inhibition. This project will yield new insight into transposon regulation, generate new research reagents and tools for studying the fertility syndrome of GD, and inform on transgenerational epigenetic processes mediated by piRNAs.